During a semiconductor packaging process, electrical wire connections are typically made between different electronic components, for example, between a semiconductor chip and a leadframe substrate. One method is to use ultrasonic welding equipment, such as an ultrasonic transducer. An ultrasonic welding operation is carried out using a transducer that is caused to vibrate at ultrasonic frequencies. The ultrasonic energy generated by the transducer is transmitted to materials to be bonded (for example, gold or aluminum wire for wire bonding and chips with gold bumps for thermosonic flip chip bonding) through a bonding tool, which is normally in the form of a wedge, capillary or collet. The ultrasonic transducer may therefore use ultrasonic energy to attach a length of bonding wire to contact surfaces or pads of the electronic components.
FIG. 1 is a side view of a conventional ultrasonic transducer 100 of the prior art. It comprises a plurality of piezoelectric elements, such as piezoelectric ceramic rings 102 disposed in a stack, for generating ultrasonic vibrations when electricity is passed through the ceramic rings 102. The ceramic rings 102 are formed with central holes and held together in compression by back masses 104. The ceramic rings 102 are connected to control circuitry and receive input signals that cause the ceramic elements to generate ultrasonic vibrations. The ultrasonic vibrations are amplified by an elongate amplifying horn 108 at the free end of which is a bonding tool, illustrated herein in the form of a capillary 110. The capillary 110 is used to apply compressive bonding force to bonding components being welded together. A barrel 106 is provided between the ceramic rings 102 and the amplifying horn 108 to allow the transducer 100 to be mounted to a wire bonding apparatus. The transducer 100 vibrates along its axis, and its amplitude of vibration is represented by a line 112 in FIG. 1. The maximum displacement amplitude is at the free end of the amplifying horn 108 where the capillary is located.
A problem with such a conventional transducer is that the bonding force bends or deforms the transducer during bonding due to a reaction force, when the bonding tool is exerting pressure on a bonding component to be welded. This bending or deformation may result in poor coplanarity, and in turn lead to uneven distribution of bonding force exerted over different locations to be bonded.
In a particular instance, during thermosonic flip chip bonding, welding is performed on the number of gold bumps on a semiconductor chip. The bonding force could be in the order of 10 kg. The huge bonding force could deform the transducer and result in poor coplanarity on the bonding tool tip. The poor coplanarity leads to an uneven distribution of bonding force among gold bumps. A significant bonding strength difference may thus result among each bonding bump.